Herbert M. Urbassek

Molecular-dynamics simulation of sputtering

Abstract A review is given on the method of molecular-dynamics computer simulation, and on the results obtained on the physics of sputtering. On the methodological side, the physical input (such as the interatomic potentials, coupling to the electronic system), the reliability, and computer time requirements of simulations are discussed. Molecular-dynamics results obtained after 1992, the time of the last review, are presented. Those results are emphasized, which are difficult to be obtained by other theoretical or computational means: sputtering from high-energy-density zones (spikes), cluster emission, formation of surface topography and their influence on sputtering, and chemical effects.

A gas-flow model for the sputtering of condensed gases

Abstract The bombardment of condensed gases by low keV ions or atoms establishes a collision cascade in the solid. We argue that in cases where the deposited energy density surpasses the critical temperature of the medium, the gas formed expands into the vacuum, increasing the sputtering yield and shifting the energy spectrum to lower energies. A simple model, based on the assumption of a collision-free molecular flow, is developed to give analytical results for the sputtering yield and the axial energy spectrum. The effects of the expansion process, of energy dissipation and of recondensation on the crater walls are discussed. Good agreement with experimental sputtering yields and energy spectra is found.

Round Robin computer simulation of ejection probability in sputtering

Abstract We have studied the ejection of a copper atom through a planar copper surface as a function of recoil velocity and depth of origin. Results were obtained from six molecular dynamics codes, four binary collision lattice simulation codes, and eight Monte Carlo codes. Most results were found with a Born-Mayer interaction potential between the atoms with Gibson 2 parameters and a planar surface barrier, but variations on this standard were allowed for, as well as differences in the adopted cutoff radius for the interaction potential, electronic stopping, and target temperature. Large differences were found between the predictions of the various codes, but the cause of these differences could be determined in most cases. A fairly clear picture emerges from all three types of codes for the depth range and the angular range for ejection at energies relevant to sputter ejection, although a quantitative discussion would have to include an analysis of replacement collision events which has been left out here.

Influence of crystal anisotropy on elastic deformation and onset of plasticity in nanoindentation: A simulational study

Using molecular-dynamics simulation we simulate nanoindentation into the three principal surfaces—the (100), (110), and (111) surface—of Cu and Al. In the elastic regime, the simulation data agree fairly well with the linear elastic theory of indentation into an elastically anisotropic substrate. With increasing indentation depth, the effect of pressure hardening becomes visible. When the critical stress for dislocation nucleation is reached, even the elastically isotropic Al shows a strong dependence of the force-displacement curves on the surface orientation. After the load drop, when plasticity has set in, the influence of the surface orientation is lost, and the contact pressure (hardness) becomes independent of the surface orientation.

Pair vs many-body potentials: Influence on elastic and plastic behavior in nanoindentation of fcc metals

Molecular-dynamics simulation can give atomistic information on the processes occurring in nanoindentation experiments. In particular, the nucleation of dislocation loops, their growth, interaction and motion can be studied. We investigate how realistic the interatomic potentials underlying the simulations have to be in order to describe these complex processes. Specifically we investigate nanoindentation into a Cu single crystal. We compare simulations based on a realistic many-body interaction potential of the embedded-atom-method type with two simple pair potentials, a Lennard-Jones and a Morse potential. We find that qualitatively many aspects of nanoindentation are fairly well reproduced by the simple pair potentials: elastic regime, critical stress and indentation depth for yielding, dependence on the crystal orientation, and even the level of the hardness. The quantitative deficits of the pair potential predictions can be traced back: (i) to the fact that the pair potentials are unable in principle to model the elastic anisotropy of cubic crystals and (ii) as the major drawback of pair potentials we identify the gross underestimation of the stable stacking fault energy. As a consequence these potentials predict the formation of too large dislocation loops, the too rapid expansion of partials, too little cross slip and in consequence a severe overestimation of work hardening.

Hydrodynamical instability of melt flow in laser cutting

A dynamic model of melt ejection by a gas jet in laser cutting is presented. The molten material is removed due to friction forces and the pressure gradient of the gas flow. The solution of the stationary equations yields the thickness of the molten layer and its velocity of flow, dependent on cutting speed, gas jet formation and the viscosities and densities of the melt and the gas. A stability analysis of the stationary flow shows instabilities for a pressure gradient controlled melt removal. It is argued that these instabilities correlate with ripple formation on the cutting surface.

On laser fusion cutting of metals

The laser fusion cutting of metals is modelled theoretically for high cutting velocities assuming that light absorption occurs according to the classical Fresnel formulae. For a beam polarised in the cutting direction, a threshold intensity for efficient cutting exists. The dependence of the cutting depth and the mean absorbed laser power on the laser intensity and the mode number is discussed. An optimal choice for the laser focus position and the beam divergence is derived.

Effect of gas‐phase collisions in pulsed‐laser desorption: A three‐dimensional Monte Carlo simulation study

The gas flow of particles laser desorbed from an elemental target into a vacuum is studied by Monte Carlo simulation. Pulsed desorption off a finite area is modeled; this is possible by using a three‐dimensional simulation algorithm. We monitor the temporal evolution of the desorption cloud and global features of the flow, such as the number of collisions occurring in the gas, and the fraction of particles backscattered to the surface. The angle and energy distribution of the desorbed particles is investigated as a function of the number of monolayers desorbed, and the laser spot width. Our results show the formation of a desorption jet, in which fast particles are focused towards the jet axis, while slow particles leave the jet at oblique angles. Many features of the particle flux may be fitted by so‐called elliptical distributions. However, these represent the velocity distribution of particles at oblique angles only poorly. Finally, we demonstrate the differences which exist between our three‐dimension...

Pressure-transmitting boundary conditions for molecular-dynamics simulations

A scheme for establishing boundary conditions in molecular-dynamics simulations that prevent pressure wave reflection out of the simulation volume is formulated. The algorithm is easily implemented for a one-dimensional geometry. Its efficiency is tested for compressive waves in Cu.

Molecular‐dynamics simulations of bulk and surface damage production in low‐energy Cu→Cu bombardment

Molecular‐dynamics simulations are employed to study in detail the effects of low‐energy (≤100 eV) bombardment of a Cu (001) surface by Cu atoms. By following the simulation up to 4 ps in real time, the end configuration of defects in the target can be observed. We present results on the vacancy and interstitial distribution in the target, the spontaneous defect recombination, the number of surface vacancies and adatoms produced, and the mixing of target atoms induced by the bombardment. Furthermore, the fate of the projectile atom−backscattering and implantation−and the sputtering behavior are investigated. The relevance of the results on the modelling of ion‐beam (assisted) deposition is discussed.